Despite all the official media, I myself find it hard to determine Birdo's gender. My 7-year-old brother actually believes Birdo is a boy! Unless I want to talk about the topic though, I go around calling Birdo an "it" to play it safe. But when I'm wanting to talk about it...
I believe in the possibility that older American media and present-day Japanese media calling Birdo/Catherine a boy. But I also believe Birdo can be a girl too, especially if we're talking about Yoshi's love interests, because I know not everyone can be accepting for such a relationship if the two were boys, you know what I mean? I wouldn't care either way, don't get me wrong, but that doesn't mean EVERYONE else thinks the same as me.